1. Field of the Invention
The present invention relates to water treatment methods, and particularly to a method for removal of heavy metals from aqueous solutions using metal-doped titanium dioxide nanoparticles, and particularly to the removal of lead, zinc and cadmium from aqueous solutions by photocatalytic degradation of heavy metals using metal-doped titanium dioxide nanoparticles.
2. Description of the Related Art
In recent years, semiconductors have gained major attention in a vast number of physical and chemical applications such as devices manufacturing and modern electronics. This is because of the unique properties of these materials, including the band gap energy, surface area, pore volume and optical properties.
Among semiconductors, titanium dioxide, which is a white pigment with band gap energy of 3.2 eV, has been used intensively in many industrial applications, such as optical coatings, optoelectronic devices manufacturing, catalyst supports and photocatalysis. This wide range of applications comes as a result of its efficient catalytic activity, photosensitivity, non-toxicity and physical and chemical stability. When irradiated with UV light with energy higher than its band gap, it ejects an electron from the valence band to the conduction band on its surface, which reacts with electron acceptor. This ejection of electrons creates a hole (h+) in the valence band that leads to formation of highly reactive hydroxyl radicals by (h+). In addition, titanium dioxide has many superior characteristics, such as UV shielding capability which makes it an excellent prominent photocatalyst.
Generally, the photocatalytic activity of titanium dioxide is affected by the surface area, pore size and volume, the phase of the solid material, and the band gap energy. Therefore, in order to enhance this photocatalytic activity, it is inevitable to increase the surface area and pore volume of titanium dioxide, where it is hard to control the band gap energy. Thus, nano-level synthesized titanium dioxide can offer a significant improvement of photocatalytic performance.
Several synthetic routes including chemical vapor deposition, hydrolysis, micro-emulsion, template hydrothermal, sputtering and sol-gel synthesis are utilized for the synthesis of nanocrystalline titanium dioxide, and the choice of any of these methods depends on the required properties of the final catalyst and its applications.
The most commonly used techniques for synthesis of titanium dioxide and other semiconductors include the following.
Chemical vapor deposition (CVD): In this process, a substrate is exposed to precursors with higher relative volatility to allow the decomposition on the surface of substrate to produce the desired materials. The main advantage of this process is the ability to produce ultra-high purity materials with a very small particle size. The main drawback of CVD is the low yield.
Sputtering: In this technique, bombardment by energetic particles is used to eject atoms from the solid target materials in a vacuum chamber to produce semiconductors. It is a very efficient method to synthesize nano film with uniform thickness, but it is a high cost process due to the sophisticated equipment required.
Flame synthesis: This process utilizes a high temperature or plasma exposure of precursors to produce small size nano particles. It is commonly used for nano coating due to the mobility of the products as they were achieved by bombardment through the flame.
Because of the drawbacks of these processes, which are the high cost and equipment, the sol-gel technique emerges as the most promising technique due to its simplicity and ability to control the final catalyst properties. This can be attained by adjusting the process parameters, such as the ratio of precursor to acid and solvent, solution temperature, and sonication time.
To make the crystalline titanium dioxide more photocatalytically active, it is required to modify it to enhance both its surface area and photoactive wave length range. To do so, the modifications may include the incorporation of other transition metals oxides into titanium dioxide matrix to promote the catalyst surface in both physical and chemical ways. The transition metals, doped onto the surface of titanium dioxide, improve its photocatalytic activity by reducing the band gap energy. Hence, they make it more active towards the visible range. In addition, these metals prevent surface agglomeration by acting as physical promoters, and may also increase selectivity toward different materials in many applications. This is due to the higher affinity and physical interaction achieved by incorporating these metals.
Many studies have emerged concerning the application of photocatalysis in the field of environmental degradation of chemical pollutants, including organic and inorganic materials, as well as dyes. Toxic metals, which include heavy metals, are individual metals and metal compounds that negatively affect human and animal health. At trace levels, many of these elements are necessary to support life. However, at elevated levels they may build up in biological systems and become a significant health hazard.
Among the class of heavy metals that are classified by Environmental Protection agency (EPA) and Agency for Toxic Substances & Disease Registry (ATSDR), the top priority list of hazardous substances for removal from water includes lead, cadmium and zinc. These heavy metals are carcinogenic and have severe effects on the vital human organs, including lungs, kidneys and blood vessels. The maximum allowable limits of these metals vary according to the local regional regulations, but in general, they are limited to less than 20 ppm in most local regulations. Therefore, removal of these heavy metals from aqueous solution is required to maintain the water quality standards.
Several technological methods could be employed for treatment of water to remove heavy metals. Among these methods are chemical precipitation, ion exchange, membrane separation and adsorption. These methods are either costly, energy consuming or produce sludge that requires further treatment. Photocatalytic application on the other hand has gained major attention in this field, as it offers an efficient removal of a majority of pollutants with low cost of processing, as well as other features, including chemical stability and non-toxicity.
In addition to the conventional elimination of toxic metals from industrial waste effluents, the application of the light-driven processes that can occur on irradiated semiconductor photocatalysts gained in interest for the recovery of precious metals. These metals are reduced on the surface of the semiconductor particle, which is subsequently extracted from the slurry by mechanical and/or chemical means.
A more recent development in photocatalysis is the development of semiconductor nanofibers. These nanofibers have advantages over the regular semiconductors, as they have large surface areas and high reactivates. It can decrease the band gap of TiO2 from 3.2 eV to less than 2.32 eV, which demonstrates higher photo-conversion efficiency by absorbing visible light at wavelengths below 535 nm.
Thus, a method for the removal of heavy metals from aqueous solutions using metal-doped titanium dioxide nanoparticles solving the aforementioned problems is desired.